In an active matrix type liquid crystal display device, on a liquid-crystal-side surface of one transparent substrate out of respective transparent substrates which are arranged to face each other while sandwiching liquid crystal therebetween, gate signal lines which extend in the x direction and are arranged in parallel in the y direction and drain signal lines which extend in the y direction and are arranged in parallel in the x direction are formed, and respective rectangular regions which are surrounded by these respective signal lines constitute pixel regions.
Each pixel region includes a thin film transistor which is driven by the supply of scanning signals from one-side gate signal line and a pixel electrode to which video signals from one-side drain signal line are supplied through the thin film transistor.
In this manner, the driving of each pixel is performed by the supply of the scanning signals and video signals, wherein the scanning signals and the video signals are respectively supplied through terminal portions of the gate signal lines and terminal portions of the drain signal lines which are formed such that these terminal portions extend to a region outside a display part which is formed of a mass of the pixel regions.
There has been known a structure in which the gate signal lines and the drain signal lines are covered with an insulation film which also has a function of a gate insulation film for the thin film transistor or a protective film which also has a function of obviating the direct contact of the thin film transistor with the liquid crystal. Here, the respective terminal portions of the gate signal lines and the drain signal lines are exposed through holes formed in the insulation film or the protective film and, at the same time, exposed surfaces of the respective terminal portions are covered with a conductive oxide such as ITO (Indium-Tin-Oxide), for example, to prevent the occurrence of a so-called electrolytic corrosion.
In such a liquid crystal display device to realize the high transmissivity, the liquid crystal display device must satisfy both of the achievement of low resistance of the signal lines and the provision of transparent electrodes as electrodes in the inside of the pixels.
Here, to consider the achievement of low resistance of the signal lines, first of all, the use of lines containing aluminum (Al) or an alloy thereof (hereinafter, also referred to as an aluminum line), for example is considered.
On the other hand, although a voltage is applied to the signal lines by a driving circuit (semiconductor integrated circuit) mounted on the liquid crystal display device, there has been a demand that the connection resistance at a connection portion (contact portion or terminal portion) must be reduced. This is because that along with the demand for high definition of display in recent years, an area of the contact portion is largely reduced and hence, the increase of the connection resistance is no more ignored. For example, a method for mounting driving circuits to the liquid crystal display device has been shifted from a conventional TCP (Tape Carrier Package) method to a COG (Chip On Glass) method and a large decrease of the area of the contact portion is apparent in the COG method.
Simultaneously, as mentioned previously, the contact portion is covered with the conductive oxide such as ITO or the like for obviating the electrolytic corrosion, to achieve both of the reduction of connection resistance with the driving circuits and the ensuring of the reliability, polycrystalline oxide (for example, p-ITO) has been generally and specifically used as the oxide.
However, it has been confirmed that the use of the aluminum lines as the signal lines and the use of the p-ITO or the like in the terminal portions for obviating the electrolytic corrosion give rise to following drawbacks.
That is, although the use of a strong acid such as HBr, for example, is indispensable at the time of processing the p-ITO by etching, the strong acid permeates defective portions of the insulation film (insulation film having also a function of the gate insulation film for the thin film transistor) and the protective film (the insulation film for obviating the direct contact of the thin film transistor with the liquid crystal) which cover the signal lines, and eventually dissolves the aluminum lines. This induces the disconnection of the signal lines, lowers the manufacturing yield factor, and becomes a cause of the increase of cost.
To cope with such a drawback, the use of amorphous ITO (a-ITO) or IZO (Indium-Zinc-Oxide) in place of p-ITO and the use of oxalic acid ((COOH)2) as an etchant for processing the amorphous ITO or the like are considered. This is because that the acidity of the oxalic acid is not strong enough to dissolve the aluminum lines.
However, the selection of a-ITO or IZO largely increases the connection resistance at the time of connection with the driving circuit and hence, it is hardly possible to obtain the original target, that is the reduction of the contact resistance.
FIG. 26 is a graph showing the respective contact resistances of p-ITO, a-ITO and IZO. The strength of respective materials when a needle is pressed to the materials is taken on the axis of abscissas and the contact resistance between each material and the needle at the time of pressing is taken on the axis of ordinates.
In the liquid crystal display device, it is considered that the pressing pressure applied to a bump of the driving circuit with respect to the signal line at the terminal portion falls substantially within a range A in the graph. In this case, it is understood that from a viewpoint of reduction of the contact resistance, the use of p-ITO as the electrolytic corrosion obviating material is extremely advantageous compared to the use of a-ITO or the IZO.
The present invention has been made under such circumstances and it is an object of the present invention to provide a liquid crystal display device which can suppress the occurrence of electrolytic corrosion at terminal portions thereof and, at the same time, can largely reduce the connection resistance between the terminal portions and other electrodes which are connected with the terminal portions.